Diamond electronic devices and methods for their manufacture

ABSTRACT

The present invention relates to a diamond electronic device comprising a functional interface between two solid materials, wherein the interface is formed by a planar first surface of a first layer of single crystal diamond and a second layer formed on the first surface of the first diamond layer, the second layer being solid, non-metallic and selected from diamond, a polar material and a dielectric material, and wherein the planar first surface of the first layer of single crystal diamond has an Rq of less than 10 nm and has at least one of the following characteristics: (a) the first surface is an etched surface; (b) a density of dislocations in the first diamond layer breaking the first surface is less than 400 cm −2  measured over an area greater than 0.014 cm 2 ; (c) a density of dislocations in the second layer breaking a notional or real surface lying within the second layer parallel to the interface and within 50 μm of the interface is less than 400 cm −2  measured over an area greater than 0.014 cm 2 ; and (d) the first surface has an R q  less than 1 nm.

The present invention relates to electronic devices fabricated indiamond, and to methods of manufacture of these electronic devices inorder to obtain high performance.

BACKGROUND OF THE INVENTION

The present generation of high frequency (HF) and microwave signals ismostly based on Si and GaAs devices. Due to physical limitations, thesedevices cannot achieve power levels higher than a few hundred watts(depending on the frequency to be amplified) in simple solid-statedevice configurations. Wide band gap materials (diamond, SiC, GaN, etc),in principle, allow for higher power amplification per unit gate lengthat microwave frequencies. This is because a larger bias voltage, andhence a larger voltage amplitude on the microwave signal, can besupported across the transistor channel region over which the current ismodulated. In effect, the higher breakdown electric field of a wide bandgap semiconductor is exploited. In microwave transistors, the ability tosupport high voltage is particularly desirable since, generally, powerhas to be transferred to a relatively high impedance (for example 50Ω)load.

The use of diamond in manufacturing transistors of various types hasbeen described in, for example, JP-A-60246627, EP 0 343 963 B1 and WO2006/117621 A1.

WO 2006/117621 A1 discloses a metal semiconductor field-effecttransistor (MESFET). The MESFET is manufactured by providing a singlecrystal diamond material substrate having a growth surface on whichfurther layers of diamond material can be deposited, depositing aplurality of further diamond layers on the substrate growth surface, andattaching appropriate contacts to the respective diamond layers, therebydefining a transistor structure. The further diamond layers deposited onthe substrate include a boron doped interface layer (a “delta-doped”layer). Such a design presents several synthesis challenges. The mainchallenge is the requirement to produce nanometer-thin boron layerswhich transition very abruptly to an intrinsic layer (e.g. a change in Bconcentration from about 10¹⁵ B atoms per cm³ to about 10²⁰ B atoms percm³ in a few nm). Growing such boron layers (delta layers) is dependentupon a number of crucial steps including substrate surface preparationfor flatness and smoothness and diamond growth conditions. In this typeof device, the holes (acting as charge carriers) are essentiallylocalised in a thin intrinsic diamond layer in the immediate vicinity ofthe boron acceptors in the delta layer.

An alternative design, described in co-pending application numberGB0701186.9 provides a structure in which the charge carriers andionised acceptors/donors are spatially separated leading to particularadvantages in terms of device manufacture and performance. This isachieved by putting a polar layer in contact with the diamond surface inorder to substantially confine the carriers in the diamond within a thindiamond surface layer adjacent to the polar layer.

Work has also taken place on diamond surface devices. These are notgenerally perceived as being practical devices in the long term, becausethey are in general intrinsically unstable, but they do offer a route tocharacterising the behaviour of diamond. A surface device utilises thefact that under certain circumstances a hydrogen terminated diamondsurface has free carriers in a surface layer formed by band bendingwhich can then be used in the fabrication of a device. The instabilityarises in these devices because further species need to be adsorbed tothe hydrogen terminated surface in order to induce the band bending, andthese species, and the hydrogen termination itself, can be lost, forexample if the device is heated.

Preparation of diamond surfaces has historically focused on providingflat surfaces. Flat surfaces in diamond can generally only be preparedin the first instance by mechanical processing. Subsequently any furthertreatment tends to roughen or pit the surface because of anisotropicbehaviour. WO 01/06633 reported that in homoepitaxial CVD diamondsynthesis there is benefit in mechanically preparing a substrate surfacewhich is flat and where the process is optimised to minimise sub-surfacedamage. Subsequently these surfaces are etched using an anisotropic etchsuch as a hydrogen etch or an oxygen etch prior to synthesis (preferablyin-situ and immediately preceding growth), and this etch, beinganisotropic reveals the sub-surface damage in the form of pits, so thatsynthesis takes place on a surface of reduced surface damage, but whichis no longer completely flat, being roughened or pitted by the etch.This relatively damage free but etch roughened surface is then suitablefor growth according to that disclosure.

WO 2006/117621 reveals that in fabrication of some electronic devicesmechanical processes can be used to obtain parallel faces to theelectronic material, and that this processing can be optimised toachieve both flatness or smoothness and the minimisation of subsurfacedamage, although the latter is not eliminated.

Electronic devices are manufactured in a number of materials. Typicallyfabrication of electronic devices comprises the preparation of asubstrate and the synthesis of one or more ‘epi’ or epitaxial layers onthis substrate. The epitaxial layers can differ from the substrate in anumber of ways:

-   -   Higher purity and/or lower dislocation content, since these can        be difficult to control in bulk grown substrate material    -   Dopant concentrations, for example the substrate can be        insulating to provide isolation, and the epilayers doped to        provide the active device regions.    -   In the case of heteroepitaxial layers, the basic material in the        epilayer can be different.

The situation in diamond is different:

-   -   The highest purity material can be grown in thick layers,        although the final surface of such thick layers is not flat.    -   Any interface, or new start of growth, in the diamond can be a        source of generation of new dislocations, so the number of        interfaces is in general minimised.    -   True single crystal diamond cannot be grown heteroepitaxially,        so a diamond single crystal substrate is always used.        Heteroepitaxial material can sometimes be described as single        crystal from, for example, visual inspection of the growth        surface, but still retains regions of crystal misoriented with        respect to one another and separated by low angle boundaries.

One area of similarity between diamond and more conventional electronicmaterials is that diamond can be doped, typically using boron. Dopedlayers are generally formed by CVD growth, generally in a separategrowth stage to the intrinsic layer.

SUMMARY OF THE INVENTION

The present invention provides a diamond electronic device comprising afunctional interface between two solid materials, wherein the interfaceis formed by a planar first surface of a first layer of single crystaldiamond and a second layer formed on the first surface of the firstdiamond layer, the second layer being solid, non-metallic and selectedfrom diamond, a polar material and a dielectric material, and whereinthe planar first surface of the layer of single crystal diamond has anR_(q) of less than 10 nm and has at least one, preferably at least two,preferably at least three, preferably all four of the followingcharacteristics:

-   -   (a) the first surface is an etched surface, preferably an        isotropically etched surface;    -   (b) a density of dislocations in the first diamond layer        breaking the surface is less than 400 cm⁻² measured over an area        greater than 0.014 cm²;    -   (c) a density of dislocations in the second layer breaking a        notional or real surface lying within the second layer parallel        to the interface and within 50 μm of the interface is less than        400 cm⁻² measured over an area greater than 0.014 cm²; and    -   (d) the first surface has an R_(q) less than 1 nm.

Features (a)-(d) refer to the preparation of the diamond surface, and itis generally preferred that the diamond surface has at least one, morepreferably 2 (two), more preferably 3 (three), more preferably all 4(four) of the characteristics (a)-(d).

In addition to having at least one of the characteristics (a) to (d),preferably the functional interface of the diamond electronic device ofthe present invention has regions with a layer of charge carriersadjacent thereto such that the charge carriers form the active devicecurrent where, in use, the charge carriers either move substantiallyparallel to the interface or the charge carriers move substantiallyperpendicular to and through the interface.

An interface prepared according to the above method will be termed a‘damage free planar interface’.

Preferably the interface formed, by a planar first surface of a firstlayer of single crystal diamond and a second layer formed on the firstsurface of the first diamond layer, is an internal interface.

In a further aspect, the present invention provides a diamond electronicdevice comprising a functional interface between two solid materials,wherein the interface is formed by a planar first surface of a firstlayer of single crystal diamond wherein the planar first surface hasbeen mechanically processed and a second layer formed on the firstsurface of the first diamond layer, the second layer being solid,non-metallic and selected from diamond, a polar material and adielectric material, and wherein the planar first surface of the firstlayer of single crystal diamond has an R_(q) of less than 10 nm andwherein the planar surface of the first layer of single crystal diamondis substantially free of residual damage due to mechanical processing.

Preferably the number density of defects revealed by a revealing etch inthe functional planar surface is less than about 100 mm², preferablyless than about 50 per mm², preferably less than about 20 per mm²,preferably less than about 10 per mm², preferably less than about 5 permm².

Preferably the planar surface of the first layer of single crystaldiamond material is prepared from a processed surface, preferably amechanically processed surface, preferably a mechanically preparedsurface.

As used herein, the term “mechanically processed” means that the surfacehas been subjected to a step involving conventional polishing andlapping techniques. As used herein, the term “mechanically prepared”refers to a surface that has been mechanically processed such that it issuitable for a specific intended purpose. This might include processingby a route optimised to minimise the amount of sub-surface damage asopposed to an arbitrary combination of lapping and polishing steps.

In a further aspect, the present invention provides a method forproducing a diamond electronic device comprising providing a diamondlayer having a thickness of greater than about 20 μm; preparing a firstsurface of the diamond layer by mechanical means to a have a surfaceroughness R_(q) of less than about 10 nm; etching the first surface ofthe diamond layer to form a planar first surface having a surfaceroughness R_(q) of less than about 10 nm; and forming a second layer onthe planar first surface of the diamond layer to form a functionalinterface between the diamond layer and the second layer, wherein thesecond layer is solid, non-metallic and selected from diamond, a polarmaterial and a dielectric material.

In a further aspect, the present invention provides a method forproducing a diamond electronic device comprising providing a diamondlayer having a thickness of greater than about 20 μm; preparing a firstsurface of the diamond layer by mechanical means to a have a surfaceroughness R_(q) of less than about 10 nm; growing a thin layer ofdiamond, preferably having a thickness of less than about 20 μm, on thefirst surface of the diamond layer to from a planar first surface havinga surface roughness R_(q) of less than about 10 nm; and forming a secondlayer on the planar first surface of the diamond layer to form afunctional interface between the diamond layer and the second layer,wherein the second layer is solid, non-metallic and selected fromdiamond, a polar material and a dielectric material.

Preferably the diamond layer is single crystal diamond.

In the context of this invention, a planar interface is an interfacewhich is not necessarily flat over large dimensions, e.g. overdimensions larger than about 1 μm, more preferably larger than about 10μm, more preferably larger than 100 about μm, more preferably largerthan about 1 mm, but on this scale may show a degree of curvature.However the interface is planar because it is free of sharp featureswhich may degrade the performance of the device by causing scattering ofthe charge carriers. In particular, the first surface of the firstlayer, and preferably the interface formed on it, preferably hasroot-mean-square roughness R_(q) of less than about 10 nm, preferably anR_(q) of less than about 5 nm, preferably an R_(q) of less than about 3nm, preferably an R_(q) of less than about 2 nm, preferably an R_(q) ofless than about 1 nm, preferably an R_(q) of less than about 0.5 nmpreferably an R_(q) of less than about 0.3 nm, preferably an R_(q) ofless than about 0.2 nm, preferably an R_(q) of less than about 0.1 nm.Furthermore, the surface of the second layer facing the first layerpreferably has an R_(q) of less than about 10 nm, preferably an R_(q) ofless than about 5 nm, preferably an R_(q) of less than about 3 nm,preferably an R_(q) of less than about 2 nm, preferably an R_(q) of lessthan about 1 nm, preferably an R_(q) of less than about 0.5 nm,preferably an R_(q) of less than about 0.3 nm, preferably an R_(q) ofless than about 0.2 nm, preferably an R_(q) of less than about 0.1 nm.

A functional interface is one which forms part of the operational designof the device, such that in the absence of the interface the design ofthe device would be different and/or its operation would besignificantly changed. More specifically, the charge carriers which are,in use, the active current of the device, move in proximity to thefunctional interface, either substantially parallel thereto orsubstantially perpendicular and therethrough.

The electronic device of this invention can comprise natural singlecrystal diamond, synthetic single crystal diamond made by highpressure-high temperature (HPHT) techniques, and synthetic singlecrystal diamond made by CVD techniques (‘single crystal CVD diamond’).Alternatively it may comprise a combination of these, for example afirst layer comprising boron doped HPHT diamond providing a firstsurface, and single crystal CVD diamond providing a second layer.

Preferably the first layer of the electronic device of this inventioncomprises single crystal CVD diamond. Preferably where the second layeris diamond this comprises single crystal CVD diamond.

Preferably, either the first layer and/or the second layer, betweenwhich there is the interface, is high purity single crystal diamond,preferably high purity single crystal CVD diamond.

The high purity single crystal diamond preferably has a total impuritycontent, excluding hydrogen and its isotopes of about 5×10¹⁸ atoms percm³ or less, preferably about 1×10¹⁸ atoms per cm³ or less, preferablyabout 5×10¹⁷ atoms per cm³ or less.

Alternatively or in addition, the high purity single crystal diamond hasa nitrogen content of about 5×10¹⁷ atoms per cm³ or less, preferablyabout 1×10¹⁷ atoms per cm³ or less, preferably about 5×10¹⁶ atoms percm³ or less, preferably about 1×10¹⁶ atoms per cm³ or less.

Alternatively or in addition, the high purity single crystal diamond hasa boron content of about 1×10¹⁷ atoms per cm³ or less, preferably about1×10¹⁶ atoms per cm³ or less, preferably about 5×10¹⁵ atoms per cm³ orless, preferably about 1×10¹⁵ atoms per cm³ or less.

The total impurity, nitrogen and boron concentrations can be measured bytechniques including secondary ion mass spectroscopy (SIMS). SIMS can beused to provide bulk impurity concentrations and to provide ‘depthprofiles’ of the concentration of an impurity. The use of SIMS is wellknown in the art, for example the measurement of boron concentrations bySIMS is disclosed in WO 03/052174.

The interface may be formed by etching or regrowth. Preferably theinterface is formed by etching, preferably by isotropic etching. Wherethe interface is formed by isotropic etching preferably it is preparedby ICP etching using a gas mixture containing a halogen and an inertgas. Preferably the halogen is chlorine and the inert gas is argon.

Advantageously, by use of the technique of isotropic etching, thesurface(s) which form the interface are etched at approximately the samerate irrespective of crystal orientation. This is particularlyadvantageous as it means that the interface may be formed from singlecrystal or polycrystalline diamond. This also means that the surface(s)can be etched without preferentially removing the damaged regions, aswould otherwise be the case were an anisotropic etch to be used. Thus,the isotropic etch removes damage from the surface without significantlyroughening the surface.

Etching

An etched surface means the removal of a minimum thickness of materialfrom the surface.

In one embodiment, an etched surface means the removal of a minimumthickness of material from an as mechanically processed surface,preferably a mechanically prepared surface, based on grit size of lastmechanical process, to provide a surface which is free or substantiallyfree of mechanical processing damage, and is also free or substantiallyfree of damage etch features.

As indicated above, an isotropically etched surface means that thesurface roughness of the surface is not substantially increased by theetch. Surface roughness measurements R_(q) ^(B) and R_(q) ^(A) are takenon the same area of the diamond. By “same area” is meant an equivalentarea as close as reasonably practical, using multiple measurements andstatistical analysis where necessary to verify the general validity ofthe measurements, as is known in the art. In particular theisotropically etched surface of the invention has a roughness R_(q) ^(A)(After the etch) and the original surface a roughness R_(q) ^(B) (Beforethe etch), such that R_(q) ^(A)/R_(q) ^(B) is preferably less than about1.5, more preferably less than about 1.4, more preferably less thanabout 1.2, more preferably less than about 1.1, and in addition, theisotropic etch preferably provides at least one, preferably at least twoof the following features:

-   -   an etched surface which is smooth and preferably smoother than        the initially prepared surface, and in particular where the        R_(q) of the etched surface (R_(q) ^(A)) is preferably less than        about 10 nm, preferably less than about 5 nm, preferably less        than about 2 nm, preferably less than about 1 nm, preferably        less than about 0.5 nm, preferably less than about 0.3 nm.    -   Removal of a thickness of material exceeding at least about 0.2        μm, more preferably at least about 0.5 μm, more preferably at        least about 1.0 μm, more preferably at least about 2 μm, more        preferably at least about 5 μm, more preferably at least about        10 μm.

Removal, by etching, of a minimum thickness of material from the asmechanically processed surface based on grit size of last mechanicalprocess, to provide a surface which is free or substantially free ofmechanical processing damage, requires the removal of sufficient depthto significantly reduce the surface damage and thus needs removal byetching of the same order of thickness as the surface damage layer.Typically surface damage layers have thicknesses in the range of about0.2 μm to about 20 μm (or thicker with very aggressive lapidarytechniques). Thus preferably the etch removes a thickness of materialfrom the surface, where the thickness of material removed is at leastabout 0.2 μm, more preferably at least about 0.5 μm, more preferably atleast about 1.0 μm, more preferably at least about 2 μm, more preferablyat least about 5 μm, more preferably at least about 10 μm. The surfacedamage layer typically has a thickness that is about the same as thesize of the largest diamond grit particle used for the last stage oflapidary processing; for example a surface scaife polished with 1-2 μmsized diamond grit will typically have a surface damage layer about 2 μmthick. Therefore, to minimise the amount of damage from lapidaryprocessing that remains after etching by the method of the invention,the amount of material removed by the method of the invention shouldpreferably be at least about 0.2 times the size of the largest gritparticles, more preferably at least about 0.5 times the size of thelargest grit particles, more preferably at least about 0.8 times thesize of the largest grit particles, more preferably at least about 1.0times the size of the largest grit particles, more preferably at leastabout 1.5 times the size of the largest grit particles, more preferablyat least about 2 times the size of the largest grit particles. After theetch, the surface of the single crystal diamond preferably has a surfaceroughness after the etch, R_(q), of less than about 10 nm, morepreferably less than about 5 nm, more preferably less than about 2 nm,more preferably less than about 1 nm, more preferably less than about0.5 nm, more preferably less than about 0.3 nm.

Where the interface is formed by etching it can extend across the wholeof a surface of the first diamond layer, or across a proportion of thesurface such as structural features etched into the surface, using knowntechniques such as photolithography, this portion of the surface thenforming the first surface.

Where the interface is formed by etching, the interface is preferably afunctional interface in the design of the electronic device, and ispreferably one of the following interfaces deemed to be an internalsurface or interface of the final device:

-   -   a diamond to diamond interface, such as intrinsic diamond to        boron doped diamond, or vice versa, or between two diamond        layers of different doping concentration, where a dopant        concentration changes across the interface by at least a factor        of about 2, preferably by at least a factor of about 5,        preferably by at least a factor of about 10, preferably by at        least a factor of about 20,    -   a diamond to diamond interface where the level of at least one        impurity changes at the interface, such that:        -   the impurity concentration in at least one layer is greater            than 10¹⁵ atoms/cm³, preferably greater than about 3×10¹⁵            atoms/cm³, preferably greater than about 10¹⁶ atoms/cm³,            preferably greater than about 10¹⁷ atoms/cm³, preferably            greater than about 10¹⁸ atoms/cm³, or        -   where the change in impurity concentration at the interface            is by at least a factor of about 5, preferably by at least a            factor of about 10, preferably by at least a factor of about            30, preferably by at least a factor of about 100, and            preferably where the impurity is other than hydrogen;    -   a diamond to non-diamond polar material interface;    -   a diamond to a non-diamond dielectric material interface.

Where the interface is formed by etching, more preferably the interfaceis functional interface in the design of the electronic device, and ispreferably one of the following interfaces deemed to be an internalsurface or interface of the final device:

-   -   a diamond to diamond interface, such as intrinsic diamond to        boron doped diamond, or vice versa, or between two diamond        layers of different doping concentration, where a dopant        concentration changes across the interface by at least a factor        of about 2, preferably by at least a factor of about 5,        preferably by at least a factor of about 10, preferably by at        least a factor of about 20;    -   a diamond to diamond interface where the level of at least one        impurity changes at the interface, such that:        -   the impurity concentration in at least one layer is greater            than 10¹⁵ atoms/cm³, preferably greater than about 3×10¹⁵            atoms/cm³, preferably greater than about 10¹⁶ atoms/cm³,            preferably greater than about 10¹⁷ atoms/cm³, preferably            greater than about 10¹⁸ atoms/cm³, or        -   where the change in impurity concentration at the interface            is by at least a factor of about 5, preferably by at least a            factor of about 10, preferably by at least a factor of about            30, preferably by at least a factor of about 100, and            preferably where the impurity is other than hydrogen;    -   a diamond to non-diamond polar material interface.

Where the interface is formed by etching, more preferably the interfaceis functional interface in the design of the electronic device, and ispreferably one of the following interfaces deemed to be an internalsurface or interface of the final device:

-   -   a diamond to diamond interface, such as intrinsic diamond to        boron doped diamond, or vice versa, or between two diamond        layers of different doping concentration, where a dopant        concentration changes across the interface by at least a factor        of about 2, preferably by at least a factor of about 5,        preferably by at least a factor of about 10, preferably by at        least a factor of about 20;    -   a diamond to diamond interface where the level of at least one        impurity changes at the interface, such that:        -   the impurity concentration in at least one layer is greater            than 10¹⁵ atoms/cm³, preferably greater than about 3×10¹⁵            atoms/cm³, preferably greater than about 10¹⁶ atoms/cm³,            preferably greater than about 10¹⁷ atoms/cm³, preferably            greater than about 10¹⁸ atoms/cm³, or        -   where the change in impurity concentration at the interface            is by at least a factor of about 5, preferably by at least a            factor of about 10, preferably by at least a factor of about            30, preferably by at least a factor of about 100, and            preferably where the impurity is other than hydrogen.

Furthermore the etched diamond surface with low R_(q) preferably issubstantially free of processing damage such that the number of defectsrevealed by the revealing etch test is less than about 100 per mm².

In the context of this invention the term ‘impurity’ refers to atomsother than Sp³-bonded carbon (that is carbon bonded as diamond) orhydrogen (and their isotopes) that are either intentionally orunintentionally present in the diamond of the invention. A dopant issuch an impurity added to modify the electronic properties of thediamond, and the material containing the dopant described as ‘dopeddiamond’. An example of an impurity which is intentionally present inthe invention is boron, which is added so as to provide a source ofcarriers and is thus a dopant. An example of an impurity which may beunintentionally present in the invention is nitrogen, which may havebeen incorporated as a result of being present in the source gases usedfor synthesis or as a residual gas in the CVD synthesis system.

Impurity concentrations can be measured by techniques includingsecondary ion mass spectroscopy (SIMS). SIMS can be used to provide bulkimpurity concentrations and to provide ‘depth profiles’ of theconcentration of an impurity. The use of SIMS is well known in the art,for example the measurement of boron concentrations by SIMS is disclosedin WO 03/052174.

Regrowth

Formation of the interface by regrowth is advantageous because it hasthe effect of distancing any damaged layer(s) from the surface(s) whichforms the functional interface(s) of the device.

Where the interface is formed by growth it can be restricted to aportion of a surface of the first diamond layer by using maskingtechniques, this portion corresponding to the first surface, or, morepreferably, it can extend across the whole of a surface of the firstdiamond layer, this whole surface forming the first surface.

As growth is a much slower process than etching, e.g. ˜1 μm/hr ascompared to ˜0.1 μm/min, there is greater scope for the control of thethickness of the layer.

In some circumstances, the technique of regrowth may be more attractivethan an etching technique, specifically where it is possible to reducethe effect of mechanical damage sufficiently by regrowth alone. Anexample of such a situation might be the deposition of a buffer layer onto a substrate where the charge carriers do not move in the bufferlayer.

An interface formed by regrowth means growing a new thin diamond layer,where the surface of this thin layer is then used as the first surfacein its as grown state.

The interface between the mechanically processed surface and theregrowth layer preferably does not itself serve an inherent part of thedevice design (or as a functional interface) other than to provide alayer of material to displace or separate an interface which is designedto act as an interface in the electronic device design (a functionalinterface) away from an interface where there is mechanical processingdamage.

Such a thin diamond layer is preferably grown by CVD synthesis, and isthin to limit the formation of macroscopic growth steps. The thicknessof this layer, grown onto a previously mechanically prepared surface, isless than about 20 μm, preferably less than about 10 μm, preferably lessthan about 3 μm, preferably less than about 1 μm, preferably less thanabout 100 nm, preferably less than about 50 nm, preferably less thanabout 20 nm, preferably less than about 10 nm.

Such a thin layer may be prepared using a number of techniques includingmonolayer growth techniques and use of off-axis surfaces to control thepropagation of surface steps, and thus retain a very flat and smoothsurface.

Where the surface upon which the thin layer is grown has Miller indicesclose to those of a {001} surface, this being the surface upon whichhomoepitaxial CVD diamond growth is most easily accomplished, the normalto the surface is preferably between 0° and about 5°, preferably betweenabout 0.5° and about 1°, of the normal to a {001} or a {111} surface.Where the surface is close to a {001} surface, the normal to the surfaceis preferably within about 10° of the great circle passing through thepole of the {001} surface and the pole of an adjacent {101} surface.

Such a thin layer may comprise high purity intrinsic diamond, morepreferably comprising high purity intrinsic diamond with materialproperties conforming to the disclosures in WO 01/96633.

Alternatively such a thin layer may comprise conductive doped diamond,for example B doped diamond.

The surface of this thin as-grown layer forms the first surface andpreferably has an R_(q) of less than about 10 nm, preferably an R_(q) ofless than about 5 nm, preferably an R_(q) of less than about 3 nm,preferably an R_(q) of less than about 2 nm, preferably an R_(q) of lessthan about 1 nm, preferably an R_(q) of less than about 0.5 nmpreferably an R_(q) of less than about 0.3 nm, preferably an R_(q) ofless than about 0.2 nm, preferably an R_(q) of less than about 0.1 nm.Thus, this surface has very low surface roughness and in addition isfree of processing damage.

The prepared surface onto which this layer may be grown could be anyform of diamond, but is preferably CVD synthetic diamond, preferablyboron doped CVD diamond.

Furthermore, where the interface is formed by regrowth, preferably theinterface is one of the following interfaces deemed to be an internalsurface or interface of the final device:

-   -   A conductive doped diamond to conductive doped diamond        interface, such as a boron doped diamond to boron doped diamond,        where both layers contain a dopant at a concentration preferably        greater than about 10¹⁷ atoms/cm³, preferably greater than about        10¹⁸ atoms/cm³, preferably greater than about 10¹⁹ atoms/cm³,        preferably greater than about 10²⁰ atoms/cm³, and preferably        where any difference in boron doping between the layers is not        relevant to device performance and the damaged layer is        essentially encapsulated in a region of conducting diamond away        from any active device interfaces. Preferably the dopant is        boron.

A diamond to diamond interface, such as intrinsic diamond to intrinsicdiamond, wherein the properties of the diamond either side of the layerare sufficiently similar for the interface not to be designed to act asan interface in the electronic device design. Preferably the intrinsicdiamond comprises high purity intrinsic diamond with material propertiesconforming to the disclosures in WO 01/96633.

More preferably, where the interface is formed by regrowth, theinterface is a conductive doped diamond to conductive doped diamondinterface, where both layers contain a dopant at a concentrationpreferably greater than about 10¹⁷ atoms/cm³, preferably greater thanabout 10¹⁸ atoms/cm³, preferably greater than about 10¹⁹ atoms/cm³,preferably greater than about 10²⁰ atoms/cm³, and preferably where anydifference in boron doping between the layers is not relevant to deviceperformance and the damaged layer is essentially encapsulated in aregion of conducting diamond away from any active device interfaces.Preferably the dopant is boron.

Combined

The techniques of etching and regrowth may be combined, such that asurface is first etched and then a thin layer regrown to form the firstsurface of the first layer and subsequently the interface. This approachis generally advantageous only if the etch has not been completed tosufficient depth to remove all mechanical processing damage. However, byuse of a combination of the two techniques, it is envisaged that it ispossible to produce an interface which has minimal surface damage. Thisis because the damage has first been removed by etching and then anyresidual damage is distanced from the functional interface by the growthof the thin diamond layer.

PREFERRED EMBODIMENTS OF THE INVENTION

It is desirable that the first layer has a low dislocation density inthe region of the first surface. In particular, it is desirable that thedensity of dislocations breaking the first surface of the first layer isless than about 400 cm⁻², preferably less than about 300 cm⁻²,preferably less than about 200 cm⁻², preferably less than about 100cm⁻², measured over an area of greater than about 0.014 cm², preferablygreater than about 0.1 cm², preferably greater than about 0.25 cm²,preferably greater than about 0.5 cm², preferably greater than about 1cm², and preferably greater than about 2 cm².

Methods of preparing and characterising diamond and diamond surfaceswith low dislocation density are reported in the prior art of WO01/96633, WO 01/96634, WO 2004/027123, and co-pending applicationPCT/IB2006/003531. The preferred methods of characterising thedislocation density are the use of a ‘revealing plasma etch’ and the useof X-ray topography.

It is further desirable that the first surface of the first layerforming one side of the interface is substantially free from damageintroduced by post-growth mechanical processing of the as-grown surfaceto a depth of at least about 1 nm, preferably at least about 2 nm,preferably at least about 5 nm, preferably at least about 10 nm,preferably at least about 20 nm, preferably at least about 50 nm,preferably at least about 100 nm, preferably at least about 200 nm,preferably at least about 500 nm. The presence of such damage, whichincludes microfractures and mechanically-generated point and extendeddefects, can have a detrimental effect on the performance of a devicethrough carrier scattering and trapping, perturbation of the localelectric field and degradation of the breakdown electric field.

In the case of diamond and in particular single crystal CVD diamond,such defects can be introduced into the material by mechanicalprocessing of the as-grown surface, such as by using conventionallapping and polishing techniques. These issues are particularly relevantto diamond in view of its hard and brittle nature, and its chemicalinertness which limits the number of chemical and physical etchingprocesses available. The requirements for processing an electronicsurface to obtain low roughness, and those for processing an electronicsurface to obtain low surface damage, are quite distinct. Thepreparation of an electronic interface showing both these features is afurther aspect of this invention.

Generally, thick layers of single crystal CVD diamond in the as-grownstate are not suitable for use as the first layer and their surfaces arenot suitable for use as the first surface because of the presence ofnon-planar features that can develop during thick growth. The presenceof non-planar features, even if they are epitaxial to the underlyingsurface, results in surfaces being present that do not have the samecrystallographic characteristics. For example, hillocks with surfacesformed by {111} planes may be present on the {001} surface. This isundesirable as in subsequent growth it can result in the presence ofregions of different growth sector and produce regions which havedifferent properties. Further, boundaries between regions of differentgrowth sectors can be the source of dislocations which are detrimentalto the electronic properties of the device. Therefore, it is desirableto ensure that the surface is flat i.e. has an Rq as defined above andis free from surface features.

Conversely, the diamond layer on which the electronic surface is to beprepared needs to sufficient rigid and robust for processing andhandling, and consequently the fabrication of an electronic deviceusually starts from a thick diamond layer. There are a number of methodsprovided in this invention of producing a suitable diamond surface fromthe as-grown surface of a thick diamond layer, which processing stepsare included in the method. In the context of this invention, a singlecrystal CVD layer is considered to be thick when its thickness exceedsabout 20 μm.

Firstly, a first surface may be prepared on the thick diamond layerusing mechanical lapping and polishing processes, which have beenoptimised for minimum surface damage by using feedback from, forexample, a revealing etch. Such a technique is described in for exampleWO 01/96633. Whilst such a surface may have a low damage level, it isunlikely to be sufficiently free of damage to obtain more than adequateperformance from the device.

The first surface may then be prepared from a processed surface,preferably from a mechanically processed surface, preferably amechanically prepared surface itself optimised for minimum surfacedamage by using the method above, by using a further processing stagecomprising chemical etch or other forms of etching, such as ion beammilling, plasma etching or laser ablation, and more preferably plasmaetching. Preferably the etching stage removes at least about 10 nm,preferably at least about 100 nm, more preferably at least about 1 μm,more preferably at least about 2 μm, more preferably at least about 5μm, more preferably at least about 10 μm. Preferably the etching stageremoves less than about 100 μm, preferably less than about 50 μm,preferably less than about 20 μm. This further processed surfacepreferably has an R_(q) of less than about 10 nm, preferably an R_(q) ofless than about 5 nm, preferably an R_(q) of less than about 3 nm,preferably an R_(q) of less than about 2 nm, preferably an R_(q) of lessthan about 1 nm, preferably an R_(q) of less than about 0.5 nmpreferably an R_(q) of less than about 0.3 nm, preferably an R_(q) ofless than about 0.2 nm, preferably an R_(q) of less than about 0.1 nm.

Alternatively, the first surface may be prepared from a processedsurface, preferably from a mechanically processed surface, preferably amechanically prepared surface itself optimised for minimum surfacedamage by using the method above, or from an etched surface such asthose described above, by growing a further thin layer of diamond on thesurface, preferably using a CVD process. Prior to deposition of thefurther thin layer of diamond (termed regrowth), the processed surfacehas an R_(q) of less than about 10 nm, preferably an R_(q) of less thanabout 5 nm, preferably an R_(q) of less than about 3 nm, preferably anR_(q) of less than about 2 nm, preferably an R_(q) of less than about 1nm, preferably an R_(q) of less than about 0.5 nm preferably an R_(q) ofless than about 0.3 nm, preferably an R_(q) of less than about 0.2 nm,preferably an R_(q) of less than about 0.1 nm. After deposition of thefurther thin layer of diamond (termed regrowth), the new as grownregrowth surface has an R_(q) of less than about 10 nm, preferably anR_(q) of less than about 5 nm, preferably an R_(q) of less than about 3nm, preferably an R_(q) of less than about 2 nm, preferably an R_(q) ofless than about 1 nm, preferably an R_(q) of less than about 0.5 nmpreferably an R_(q) of less than about 0.3 nm, preferably an R_(q) ofless than about 0.2 nm, preferably an R_(q) of less than about 0.1 nm.

Where the first surface is prepared by plasma etching, preferably theetching is achieved by ICP etching, preferably using a gas mixturecontaining a halogen and an inert gas, preferably where the inert gas isargon, and preferably where the halogen is chlorine.

The electronic device may be a 2-terminal device, such as a diode.

The electronic device may have at least 3 terminals, such as a3-terminal transistor.

The electronic device is preferably a transistor, preferably a fieldeffect transistor.

In one embodiment of the present invention, the electronic devicecomprises a functional interface between two solid materials, whereinthe interface is formed by a planar first surface of a first layer ofsingle crystal diamond, wherein the planar first surface has preferablybeen mechanically processed and subsequently isotropically etched and asecond layer formed on the first surface of the first diamond layer, thesecond layer being solid, non-metallic and selected from diamond, apolar material and a dielectric material, and wherein the planar firstsurface of the first layer of single crystal diamond has an R_(q) ofless than about 10 nm, preferably an R_(q) of less than about 5 nm,preferably an R_(q) of less than about 3 nm, preferably an R_(q) of lessthan about 2 nm, preferably an R_(q) of less than about 1 nm, preferablyan R_(q) of less than about 0.5 nm preferably an R_(q) of less thanabout 0.3 nm, preferably an R_(q) of less than about 0.2 nm, preferablyan R_(q) of less than about 0.1 nm.

In another embodiment of the present invention, the diamond electronicdevice comprises a functional interface between two solid materials,wherein the interface is formed by a planar first surface of a firstlayer of single crystal diamond and a second layer formed on the firstsurface of the first diamond layer, the second layer being solid,non-metallic and selected from diamond, a polar material and adielectric material, and wherein the planar first surface of the firstlayer of single crystal diamond has an R_(q) of less than about 1 nm andwherein the first surface of the diamond layer is a surface of a diamondlayer, preferably having a thickness of less than about 20 μm,preferably less than about 10 μm, preferably less than about 3 μm,preferably less than about 1 μm, preferably less than about 100 nm,preferably less than about 50 nm, preferably less than about 20 nm,preferably less than about 10 nm, grown on a single crystal diamondlayer.

In another embodiment of the present invention, the electronic devicecomprises a functional interface between two solid materials, whereinthe interface is formed by a planar first surface of a first layer ofsingle crystal diamond, wherein the planar first surface has preferablybeen mechanically processed and subsequently isotropically etched and asecond layer formed on the first surface of the first diamond layer, thesecond layer being solid, non-metallic and selected from diamond, apolar material and a dielectric material, and wherein the planar firstsurface of the first layer of single crystal diamond has an R_(q) ofless than about 10 nm, preferably an R_(q) of less than about 5 nm,preferably an R_(q) of less than about 3 nm, preferably an R_(q) of lessthan about 2 nm, preferably an R_(q) of less than about 1 nm, preferablyan R_(q) of less than about 0.5 nm preferably an R_(q) of less thanabout 0.3 nm, preferably an R_(q) of less than about 0.2 nm, preferablyan R_(q) of less than about 0.1 nm and wherein the first surface of thediamond layer is a surface of a diamond layer, preferably having athickness of less than about 20 μm, preferably less than about 10 μm,preferably less than about 3 μm, preferably less than about 1 μm,preferably less than about 100 nm, preferably less than about 50 nm,preferably less than about 20 nm, preferably less than about 10 nm grownon a single crystal diamond layer.

Defining Measurement Techniques

For the purposes of this invention the roughness of a surface isdescribed by its R_(q) value. R_(q) is also known as the ‘root meansquare’ (or RMS) roughness. R_(q) is defined as the square root of themean squared deviations from the centre-line or plane of the surfaceprofile:

R _(q)=√((y ₁ ² +y ₂ ² + . . . +y _(n) ²)/n)

where y₁ ² etc are the squared deviations from the centre-line or planeof the surface profile and n is the number of measurements.

A surface may also be quantified by its R_(a) value (also referred as‘average roughness’ or ‘centre line average’):

R _(a)=(|y ₁ |+y ₂ |+ . . . |y _(n)|)/n

where |y₁| etc are the moduli of the deviations from the centre-line orplane of the surface profile and n is the number of measurements.

For a surface with a Gaussian distribution of deviations from thecentre-line or plane of the surface profile, the value ofR_(q)=1.25×R_(a).

R_(a) and R_(q) may be measured along lines (a one-dimensionalmeasurement) or over areas (a two-dimensional measurement). An areameasurement is essentially a series of parallel line measurements.

For the purposes of this invention the R_(q) value is normally measuredover a 1 μm by 1 μm area or 2 μm by 2 μm area using a scanning probeinstrument such as an atomic force microscope (AFM). In certaincircumstances, it is considered more appropriate to measure the R_(q)using a stylus profilometer over a 0.08 mm scan length (or over whateverlength is standard within the art for the roughness of the surface).

The extent of sub-surface damage can be revealed and quantified using adeliberately anisotropic thermal revealing etch. The revealing etchpreferentially oxidises regions of damaged diamond and therefore allowssuch regions to be identified and thereafter quantified. Regionscontaining sub-surface damage from mechanical processing are typicallydarkened or even blackened by the revealing etch.

The revealing etch consists of:

-   -   (i) examining the surface at a magnification of 50 times using        reflected light with a typical metallurgical microscope to        ensure that there are no surface features present,    -   (ii) exposing the surface to an air-butane flame thereby raising        the diamond surface to a temperature of typically about 800° C.        to about 1000° C. for a period of about 10 seconds,    -   (iii) examining the surface at a magnification of 50 times using        reflected light with a typical metallurgical microscope and        counting the damage features revealed by the revealing etch, in        the manner described below, to determine their number density,    -   (iv) repeating steps (ii) and (iii) and comparing the measured        density of defects with that of the previous cycle until the        following condition is met: if the number density of defects        counted is less than or equal to 150%, preferably less than or        equal to 120%, of the number density determined in the previous        cycle, then all the defects are deemed to be revealed and the        measurement recorded is the average of the measurements of the        last two cycles, if not the cycle is repeated again.

The number density of defects is measured by the following method:

-   -   (i) the defects to be counted are those defects visible at a        magnification of 50 times with a typical metallurgical        microscope which fall totally or partially within a rectangular        area 1 mm×0.2 mm projected onto the surface being characterised,    -   (ii) the area is selected at random over the surface or portion        of the surface to be characterised and randomly oriented,    -   (iii) the defects are counted in a minimum of 5 such areas,    -   (iv) the number density of defects is calculated by dividing the        total number of defects counted by the total area examined to        give a number density in defect per mm².

To measure the number density of defects in areas less than 1 mm² theabove method is adapted by completing the defect count over the wholearea as a single measurement.

For the surface to be considered to be substantially free of residualdamage due to mechanical processing the number density of defectsrevealed in a surface of single crystal CVD diamond prepared by themethod of the invention is less about 100 per mm², preferably less thanabout 50 per mm², preferably less than about 20 per mm², preferably lessthan about 10 per mm², preferably less than about 5 per mm².

Methods of preparing and characterising diamond and diamond surfaceswith low dislocation density are reported in the prior art of WO01/96633, WO 01/96634, WO 2004/027123, and co-pending applicationPCT/IB2006/003531. The preferred methods of characterising thedislocation density are the use of a ‘revealing plasma etch’ and the useX-ray topography.

As used herein, the term “about x” is intended to include the value xitself.

EXAMPLE

By way of example, in the case of a diode, preferably the damage freeinterface functional interface is formed between doped conductingdiamond and intrinsic diamond, and is formed by one of the followingmethods:

-   -   By regrowth, wherein the boron doped layer is formed, and a        planar surface formed on the doped diamond by lapidary or        mechanical processing. A further thin B doped layer is then        grown onto this layer, preferably using growth conditions        selected to minimise roughening and preferably keeping the layer        sufficiently thin and in the thickness range 10 nm-20 μm, more        preferably in the thickness range 100 nm-10 μm, more preferably        in the thickness range 1 μm-10 μm to minimise roughening, and        thus encapsulating the surface with mechanical damage between        two regions of doped conducting diamond. Then a high purity        intrinsic diamond layer is grown onto the regrown layer surface,        this layer preferably comprising high purity intrinsic diamond        with material properties conforming to the disclosures in        WO01/96633 thus forming a damage free interface between the thin        doped conducting diamond layer and a further layer of intrinsic        diamond which is displaced from the damage layer encapsulated        within the boron doped layer.    -   By etching, wherein the conducting doped diamond layer is        formed, and a planar surface formed on the doped diamond by        lapidary or mechanical processing. This surface is then etched,        preferably using a plasma etch, more preferably an        Argon/Chlorine plasma etch. Optionally, a further thin B doped        layer may be regrown onto this layer, preferably using growth        conditions selected to minimise roughening and preferably        keeping the layer sufficiently thin and in the thickness range        10 nm-20 μm, more preferably in the thickness range 100 nm-310        μm, more preferably in the thickness range 1 μm-10 μm to        minimise roughening. Preferably this optional layer is not used.        Then a high purity intrinsic diamond layer is grown onto the        etched surface, or optional regrown layer surface, this layer        preferably comprising high purity intrinsic diamond with        material properties conforming to the disclosures in WO        01/96633, thus forming a damage free interface between the        conducting diamond layer and a further layer of intrinsic        diamond.

Alternatively the diode structure above may be formed between a heavilyboron doped layer providing a highly conductive layer, and a lightlyboron doped layer providing the reverse voltage hold-off.

In the case of a transistor, preferably the damage free interface isformed by etching or regrowth so that the damage free surface isprepared in the intrinsic diamond. Preferably the damage free surface isparallel to the primary current flow in the device, with this currentflow taking place primarily in the intrinsic diamond layer adjacent tothe damage free surface, and that current flow is in close proximity tothe interface, typically less than 1 μm and more typically less thanabout 100 nm. Thus, there is a 2-dimensional charge carrier gas, wherethe meaning of the term “2-dimensional charge carrier gas” is as isnormally understood in the art, present in the intrinsic diamondadjacent to the damage free surface.

It will of course be understood that the present invention has beendescribed above purely by way of example, and that modifications ofdetail can be made within the scope of the invention as defined by theclaims.

1. A diamond electronic device comprising a functional interface betweentwo solid materials, wherein the interface is formed by a planar firstsurface of a first layer of single crystal diamond and a second layerformed on the first surface of the first diamond layer, the second layerbeing solid, non-metallic and selected from diamond, a polar materialand a dielectric material, and wherein the planar first surface of thefirst layer of single crystal diamond has a surface roughness R_(q) ofless than 10 nm and has at least one of the following characteristics:(a) the first surface is an etched surface; (b) a density ofdislocations in the first diamond layer breaking the first surface isless than 400 cm⁻² measured over an area greater than 0.014 cm²; (c) adensity of dislocations in the second layer breaking a notional or realsurface lying within the second layer parallel to the interface andwithin 50 μm of the interface is less than 400 cm⁻² measured over anarea greater than 0.014 cm²; and (d) the first surface has an R_(q) lessthan 1 nm.
 2. A diamond electronic device according to claim 1, whereinthe planar first surface of the first layer of single crystal diamond isa mechanically processed surface.
 3. A diamond electronic devicecomprising a functional interface between two solid materials, whereinthe interface is formed by a planar first surface of a first layer ofsingle crystal diamond wherein the planar first surface has beenmechanically processed, and a second layer formed on the first surfaceof the first diamond layer, the second layer being solid, non-metallicand selected from diamond, a polar material and a dielectric material,and wherein the planar first surface of the first layer of singlecrystal diamond has a surface roughness R_(q) of less than 10 nm andwherein the planar surface of the first layer of single crystal diamondis substantially free of residual damage due to mechanical processing.4. An electronic device according to claim 2, wherein the number densityof defects revealed by a revealing etch in the functional planar surfaceis less than 100 per mm²
 5. A diamond electronic device according toclaim 1, wherein the first surface of the first diamond layer is anetched surface.
 6. A diamond electronic device according to claim 5where the etched first surface is an isotropically etched surface.
 7. Anelectronic device according to claim 6, wherein the first surface hasbeen isotropically etched using a gas mixture comprising a halogen andan inert gas.
 8. An electronic device according to claim 6, wherein theetch removed at least 0.2 μm from the first surface of the first diamondlayer.
 9. An electronic device according to claim 8, wherein the etchremoved at least 0.2× the largest grit particle size used in the laststage of lapidary processing. 10-16. (canceled)
 17. A diamond electronicdevice according to claim 1 wherein the first surface of the diamondlayer is a surface of a diamond layer grown on a single crystal diamondlayer.
 18. A diamond electronic device according to claim 17 wherein thegrown diamond layer has a thickness of less than 20 microns.
 19. Adiamond electronic device according to claim 1 wherein the first surfaceof the first diamond layer is substantially free from damage introducedby post-growth mechanical processing of an as-grown surface to a depthof at least 1 nm. 20-24. (canceled)
 25. A diamond electronic deviceaccording to claim 1 wherein diamond on one side of the interface isintrinsic diamond and the diamond on the other side of the interface isboron-doped diamond. 26-31. (canceled)
 32. A method for producing adiamond electronic device comprising: (i) providing a diamond layerhaving a thickness of greater than about 20 μm; (ii) preparing a firstsurface of the diamond layer by mechanical means to a have a surfaceroughness R_(q) of less than about 10 nm; (iii) etching the firstsurface of the diamond layer to form a planar first surface having asurface roughness R_(q) of less than about 10 nm; and (iv) forming asecond layer on the planar first surface of the diamond layer to form afunctional interface between the diamond layer and the second layer,wherein the second layer is solid, non-metallic and selected fromdiamond, a polar material and a dielectric material.
 33. A method forproducing a diamond electronic device comprising: (i) providing adiamond layer having a thickness of greater than about 20 μm; (ii)preparing a first surface of the diamond layer by mechanical means to ahave a surface roughness R_(q) of less than about 10 nm; (iii) growing athin layer of diamond, preferably having a thickness of less than about20 μm, on the first surface of the diamond layer to from a planar firstsurface having a surface roughness R_(q) of less than about 10 nm; and(iv) forming a second layer on the planar first surface of the diamondlayer to form a functional interface between the diamond layer and thesecond layer, wherein the second layer is solid, non-metallic andselected from diamond, a polar material and a dielectric material. 34.The method according to claim 32, wherein the etch is an isotropic etch.35. The method according to claim 34, wherein in step (iii), at leastabout 10 nm is removed.
 36. The method according to claim 32, whereinthe diamond layer is a boron-doped single crystal diamond. 37.(canceled)
 38. The method according to claim 33, wherein the thin layerof diamond on the first surface has a thickness of less than 20 μm. 39.The method according to claim 34, wherein a gas mixture comprising ahalogen and an inert gas is used in the isotropic etch.